1. Field of the Invention
The present invention relates to an on-vehicle communication system that performs communication with plural electronic apparatuses mounted on a vehicle, specifically relates to an on-vehicle communication system that performs optical communication, an optical communication harness and an optical distribution apparatus.
2. Description of Related Art
Many electronic apparatuses are conventionally mounted on a vehicle (e.g., car), and are connected with communication lines to each other. These electronic apparatuses exchange information with each other and work together to control functions regarding travel of the car and to control functions regarding passenger comfort of the car. The controller area network (CAN) is well utilized as the communication standard for implementing the communication with the electronic apparatuses mounted on the car (see non-patent documents 1 and 2).
In addition, it tends to provide a recent car with more electric apparatuses. Thus, it is required to connect many electronic apparatuses to each other with one communication line (CAN bus), and to implement intercommunications between many electronic apparatuses. However, the ringing phenomenon is often caused when many electronic apparatuses are connected to the CAN bus. In other words, the communication failure may be often caused. Hence, these circumstances limit the number of electronic apparatuses connectable to the CAN bus. Furthermore, it is known that the communication failure is caused by the disturbance noise superimposed on the CAN bus, especially near an electronic apparatus and communication line utilizing high-voltage signals, in an area where many electronic apparatuses and communication lines are mounted tightly, e.g., the engine room of a car.
For these problems, it is considered to utilize the optical communication with an optical cable that is insensitive to the electromagnetic noise. It is proposed to partially utilize the optical communication in the field of car industry, too (see patent document 1).
It may be considered that the optical communication is utilized for not only partial communications but also all communications between the electronic apparatuses on a car (i.e., on-vehicle electronic apparatuses). It is preferred to enable the communication system based on this consideration without completely change the conventional electronic apparatus. Thus, such a communication system is preferred to enable utilizing the communication functions based on the conventional communication protocol designed for each electronic apparatus.
Based on the CAN protocol, the CAN bus is configured with a twisted-pair cable transmitting a differential signal and is connected to plural electronic apparatuses. Each electronic apparatus transmits and receives a digital signal represented by the differential signal. Because the CAN is a protocol for the serial communication, only one electronic apparatus among the plural electronic apparatuses connected to the CAN bus can perform the transmission processing and the other electronic apparatuses must wait until said one electronic apparatus has completed the transmission processing. When some electronic apparatuses simultaneously perform the transmission processing (i.e., when the communication collision occurs), each electronic apparatus performs the communication arbitration processing and the transmission processing is performed on a communication having the highest priority.
In order to perform the arbitration processing for addressing the communication collision, each electronic apparatus detects the signal level of the CAN bus at the same time outputting the transmission signal to the CAN bus. In the case that the signal level is changed (from the recessive to the dominant) after each electronic apparatus outputs the transmission signal, the each electronic apparatus determines that the communication collision occurs and stops performing the transmission processing. The dominant signal has a higher priority than the recessive signal. Thus, the electronic apparatus that has transmitted the dominant signal can continuously perform the transmission processing even when the communication collision occurs.    Patent Document 1: Japanese Patent Application Laid-Open No. 2008-219353    Non-Patent Document 1: ISO 11898-1; 2003 Road vehicles—Controller area network (CAN)—Part1; Data link layer and physical signaling    Non-Patent Document 2: ISO 11519-1; 1994 Road vehicles—Low-speed serial data communication—Part1; General and definitions